ADSORPTION GEOMETRY OF OH ADSORBED AT F-CENTERS ON A NACL(100) SURFACE

The orientation of molecular H2O in a bilayer on defect-free NaCl(100) and of chemisorbed OH adsorbed at F centers on the (100) surface of N aCl was investigated using angle-resolved O K edge near-edge x-ray-abs orption fine-structure spectroscopy (NEXAFS). High-quality single crys talline thin films of NaCl(100) of about four monolayers thickness wer e grown on Ge(100). These films were bombarded with 250 eV electrons s o that F and F' centers of a maximum density of about 10% of a monolay er were created. Whereas on the defect-free surface only molecular wat er is adsorbed, dissociation occurs at the F centers and OH- is bound. NEXAFS of molecular water shows almost no angular dependence even in the monolayer. This finding is consistent with formation of a hydrogen bonded bilayer of water suggested earlier. The OH species formed at t he F centers is found to be inclined with respect to the surface norma l by 39 degrees +/- 4 degrees. An adsorption model is proposed.